1. Field of the Invention
The present invention concerns a process for preparing tertiary alkyl ether products which are used, in particular, as a components of motor fuels. The products contain, for instance t-amyl methyl or t-amyl ethyl ethers and possibly heavier tertiary alkyl ethers. According to the process, the isoolefins, in particular the C.sub.5- C.sub.7 isoolefins of the feedstock are reacted with a suitable alkanol for preparing the corresponding ethers. These ethers are removed together with the bottoms product of the distillation-reaction system and, if necessary, they are further processed in order to prepare a motor fuel component. Unreacted alkanol is removed with the overhead product of the distillation.
2. Description of Related Art
In order to improve the anti-knocking characteristics of motor fuels without using organolead compounds, and in order to reduce the concentration of detrimental components in the exhaust gases, tertiary alkyl ethers are added to the fuels. The oxygen-containing ether group of these compounds has been found to improve the combustion process in a favourable way as far as the afore-mentioned aspects are concerned. Suitable alkyl tert-alkyl ethers are methyl t-butyl ether (MTBE), ethyl t-butyl ether (ETBE), t-amyl methyl ether (TAME), t-amyl ethyl ether (TAEE) and t-hexyl methyl ether (THME), just to mention a few examples. These ethers are prepared by etherification of a monovalent aliphatic alcohol with an isoolefin. The reaction can be carried out in a fixed bed reactor, in a fluidized bed reactor, in a tubular reactor or in a catalytic distillation column.
In a fixed bed reactor, the feed components are reacted in the presence of a solid catalyst particles, said catalyst particles being contained in a layer which remains unmixed, because the liquid flow rates are so low that the catalyst particles do not separate from each other. They form a so-called fixed bed. On the other hand, in a fluidized bed reactor, the flow rate of the liquid phase is so high that the catalyst particles float separately in the fluidized bed of the reactor.
When the etherification is carried out in a catalytic i.e. reactive distillation process, the catalyst particles can form a fixed or fluidized bed in the column. The particular benefit which can be obtained by the catalytic distillation process is that the reaction and the separation of the products take place in the same vessel.
The etherification reaction is an exothermic equilibrium reaction, and the maximum conversion is determined by the thermodynamic equilibrium of the reaction system. Typically, by carrying out reaction and separation in one and the same reactive distillation column, it is possible to obtain an about 90% conversion in the case of TAME, whereas only a 65 to 70% conversion is obtainable in a fixed bed reactor.
Ion exchange resins can be used as catalysts. Generally the resin used comprises a sulfonated polystyrene/divinyl benzene based cation exchange resin (sulfonated polystyrene cross-linked with divinylbenzene) having particle sizes in the range from 0.1 to 1 mm.
Commercially there are two alternative TAME processes available. The first one comprises fixed bed reactors, columns for product separation distillation and a methanol separation unit. The other alternative differs from the first one in the sense that the product distillation is replaced by a catalytic distillation unit, which substantially improves the TAME conversion.
In a third completely novel process alternative, which is described in our international patent application WO 93/19031 the second alternative mentioned above has been modified by transferring the catalyst from the inside of the distillation column into a separate external reactor which is being fed from the product separation distillation unit. The side reactor product is recycled back to the same product separation distillation unit.
Ethers heavier than TAME can also be produced by all of the above mentioned processes. The process described in our international patent application mentioned above can also be used for preparing other lower alkyl ethers, such as t-amyl ethyl ether (TAEE) and the corresponding heavier ethyl ethers.
The prior an processes are hampered by certain problems. Thus, the overhead product of the product distillation unit of the TAME processes contains large amounts of light hydrocarbons and, for this reason, also so much unreacted methanol that the overhead product cannot be used in an alkylation unit or directly as a gasoline component. The methanol must be removed first which is the reason why a separate methanol separation unit has to be included in the process. The methanol separation generally comprises extraction with water and methanol-water distillation.
Similar problems are encountered with the other alkanols.